This application claims Paris Convention priority of DE 100 59 585.5 filed Nov. 30, 2000 the complete disclosure of which is hereby incorporated by reference.
The present invention concerns an electrical circuit for pulse modulation of a carrier signal. The circuit comprises a pulse generator for generating a current pulse and at least one diode switch which can be controlled by the current pulse. The invention also concerns a method for pulse modulation of a carrier signal by means of at least one diode switch. A current pulse is thereby generated which is used to control the at least one diode switch.
Circuits of the above-mentioned type are used e.g. for pulse modulation of a radio-frequency carrier signal which are used e.g. for distance radar systems in motor vehicles. The diode switch is triggered by the current pulse generated by the pulse generator and switches a radio-frequency (RF) signal for a very short time period (0.1 to 10 ns).
The function of a diode switch is explained in more detail below with reference to a conventional electric circuit shown in FIG. 2. A typical diode switch 9 comprises a signal line 1 and a lambda/4 stub lead which branches off therefrom. An oscillator 3, which provides a radio-frequency carrier signal 4, is disposed at the input of the signal line 1. A control voltage source U is connected to the stub lead 2 and the control voltage U is applied to and removed from the stub lead 2 by closing and opening a switch 7. Repeated opening and closing of the switch 7 pulse-modulates the carrier signal 4 and the pulse-modulated radio-frequency carrier signal 6 is available at an output 5 of the signal line 1.
When a diode 8, disposed at the end of the stub lead 2, is conducting (switch 7 is closed), a signal can pass from the oscillator 3 to the output 5 of the diode switch 9. When the diode blocks (switch 7 is open) no signal can pass from the oscillator 3 to the output 5. The lambda/4 stub lead 2, which is connected to the diode 8, xe2x80x9cinvertsxe2x80x9d the behavior of the diode 8. During open-circuit operation (diode 8 blocks) and in case of a short-circuit (diode 8 is conducting), the signal in the stub lead 2 is reflected. This produces destructive overlapping during open-circuit operation (incoming and reflected signal are phase-shifted relative to one another by half a period) after the lambda/4 stub lead 2. In case of a short-circuit, the signals overlap constructively after the lambda/4 stub lead 2 (incoming and reflected signal are not phase-shifted relative to one another).
Diode 8 is usually a PIN diode. A PIN diode permits switching times (shortest pulse which can be generated at the output 5) of longer than 10 ns. The very small PIN diode resistance in the switched state is advantageous. To achieve considerably faster switching times, a Schottky diode can be used instead of the PIN diode. This permits switching times in the sub-ns range.
The considerably higher dynamic resistance of the Schottky diode has, however, disturbing effects. The dynamic resistance of the diode determines the insertion attenuation of the diode switch (i.e. the losses). The parasitic or stray quantities, in particular the capacitance, determined the maximum achievable isolation of the diode switch 9. The minimum achievable performance loss of the diode switch 9, i.e. with conducting diode 8, is determined by the dynamic resistance of the diode 8. The capacitance also permits small current flow to ground in the blocking state of the diode 8. The isolation of the diode switch 9 can be doubled by using a second diode connected in parallel to the first diode 8. However, this also doubles the minimum achievable power loss for the switched diode switch 9.
The circuit shown in FIG. 2 has the additional disadvantage that the envelope 6a of the pulse-modulated radio-frequency carrier signal 6 present at the output 5 of the diode switch 9 does not have a clean, square behavior. Rather, the dependence of the envelope is exponential (exe2x88x92t). When using the known circuit e.g. in a distance radar system, the position resolution obtained can be higher the shorter the emitted pulses. The shorter pulses, however, also contain reduced signal energy which renders the signal particularly susceptible to disturbances. For this reason, it is important, in particular with very short pulses, that the emitted signal, i.e. the pulse-modulated signal 6 present at the output 5 of the diode switch 9 has a clean, preferably square envelope 6a. 
In view of these considerations, it is the object of the present invention to provide a circuit and a method for modulating a carrier signal with as short pulses as possible having a clean, square envelope.
To solve this object, the invention proposes, on the basis of the electrical circuit of the above-mentioned type, that the circuit has at least one chargeable capacitive element whose discharge current controls the at least one diode switch.
The capacitive element is formed e.g. as a capacitor. It can be one individual capacitor or a series connection, a parallel connection or a combination of series and parallel connections of several capacitors. The inventive electric circuit permits modulation of a carrier signal with very short pulses having a particularly clean, square envelope. Moreover, the inventive circuit can be realized at particularly low costs.
The at least one capacitive element is initially charged through a charging resistance. Thereafter, the at least one capacitive element can be discharged at any desired point in time. The discharge current thereby controls the at least one diode switch of the electric circuit. Rapid discharge of the capacitive element produces very short current pulses. A discharge current thereby flows only until the at least one capacitive element has been discharged. The charging resistance is thereby dimensioned such that after discharging, the or each capacitive element can nearly completely recharge prior to the next current pulse.
An advantageous further development of the present invention proposes that the circuit has several diode switches with stub leads connected in parallel. Each stub lead branch has one diode, to ensure that the discharge current flows in one direction only. The maximum achievable isolation of the diode switch can be considerably increased through parallel connection of several diodes.
In another advantageous further development of the present invention, the circuit comprises at least one semiconductor switch connected in series with the/each diode switch, wherein the charged capacitive element can be discharged by a control pulse via the/each semiconductor switch and the/each diode switch after controlling the at least one semiconductor switch. The at least one capacitive element is thus discharged by controlling a semiconductor switch with a control pulse. The control pulse merely serves for triggering the discharge process. It may even last longer than the current pulse. As soon as the capacitive element is discharged, the current pulse also terminates, irrespective of whether or not the control pulse is still present on the semiconductor switch.
The/each semiconductor switch is preferably formed from a transistor and, advantageously, from a field effect transistor (FET). An FET has nearly ohmic behavior in the switched state.
In a preferred embodiment of the present invention, the circuit comprises two semiconductors, wherein the control pulse can be applied to a control contact of one semiconductor switch and a variable direct voltage is applied to a control contact of the other semiconductor switch. Application of the control pulse to the one semiconductor switch initiates the discharging process of the capacitive element. The minimum resistance in the conducting state of the one semiconductor switch can be adjusted through the variable direct voltage which is applied to the other semiconductor switch. The larger the resistance of the semiconductor switch in the conducting state, the longer it takes until the at least one capacitive element has been completely discharged to prolong the generated current pulse and the pulses which are modulated on the carrier signal by the inventive circuit.
The variable direct voltage permits adjustment of the spatial resolution or signal energy of emitted pulses e.g. in distance radar systems. Longer pulses of higher energy can be emitted for scanning remote regions. For scanning closer regions, shorter pulses can be emitted. The short pulses have a higher spatial resolution to permit safe and reliable detection of even small obstacles.
The two semiconductor switches are preferably formed as a field effect transistor (FET) comprising two gates. Such FETs are also called Dual-Gate-FETs. The variable direct voltage permits variation of the maximum drain current of the Dual-Gate-FET and hence of the discharge time of the capacitive element and the pulse duration of the pulse-modulated carrier signal. The drain current must have a minimum magnitude which permits control of all diodes of the diode switch.
In another advantageous further development of the present invention, a semiconductor unit is disposed in a branch of the circuit through which the capacitive element is charged and is switched through during the charging process. The semiconductor unit is preferably a diode. The at least one capacitive element can be charged during the charging process in the usual manner via a semiconductor unit switched in this fashion. During the discharging process, the semiconductor unit blocks and causes the entire discharge current to flow through the diode switch and not through the charging resistance.
In another advantageous further development of the present invention, the circuit comprises means for interrupting the charging process of the at least one capacitive element for at least as long as the control pulse is applied to at least one semiconductor switch. The charging signal for the at least one capacitive element and the control signal for the semiconductor switch are therefore never simultaneously HIGH. This further development prevents residual current from flowing through the charging resistance after the discharging process. If the charging signal is at LOW, discharge of the at least one capacitive element via the charging resistance can be prevented by the semiconductor unit disposed in the branch of the circuit via which the at least one capacitive element is charged.
In a preferred embodiment of the present invention, the interrupting means comprise delaying members for delaying the rising side of the control pulse and the rising side of the charging signal. The charging signal is usually at HIGH. The control signal is usually at LOW except during the control pulses. During the control pulses, the control signal is at HIGH. In a first step, the inverted control signal is used as the charging signal. The delaying members take into account inclined signal sides to ensure that, during the control pulse, the charging signal is at LOW. A first delaying member delays the rising side of the control signal, while the charging signal is changed from HIGH to LOW using the rising side of the undelayed control signal. A second delaying member delays the rising side of the charging signal which then changes from LOW to HIGH with a temporal shift relative to the falling side of the control signal. The charging signal of this embodiment is safely at LOW as long as the control signal is at HIGH.
In a further preferred embodiment of the invention, an increase in isolation of the diode switch is achieved by disposing at least one semiconductor unit, switched through during the discharge process, in series with the/each stub lead of the at least one diode switch. The semiconductor unit is preferably a diode.
In another preferred further development of the present invention, the circuit comprises an inverting branch connected in parallel with the at least one branch connected to the/each diode switch. The output of the at least one diode switch is terminated by a resistance which corresponds to the system resistance. This prevents reflection of the signal present at the output. When the at least one diode switch is switched, the applied power is terminated by the output. If the diode switch is open, no power is terminated by the output, rather, in this further development, the power is terminated by the inverting branch. The power terminated by the inverting branch is not utilized. However, termination of the power prevents the unused power from being reflected at the output of the at least one diode switch to prevent negative effects on the oscillator producing the carrier signal. The reflected power could cause fluctuations in the oscillator frequency. For use of the circuit in distance radar systems, the spatial resolution and reliability could thereby be impaired. Termination of the unused power curtails the disturbances in the at least one diode switch.
The inverting branch preferably comprises a length which is larger by lambda/4 compared to the branches with the at least one diode switch. When using a diode switch with a lambda/4 stub lead, the inverting branch is approximately designed as a lambda/2 line. To provide useful cooperation between the two branches, i.e. such that the oscillator always sees the system resistance at the input, the two branches must be mutually adjusted. Towards this end, the two branches are operated separately. Both branches are designed such that they have as low a reflection factor as possible when switched.
In another preferred further development of the present invention, a series connection of an inverting unit with a wavelength-selective reflecting unit (so-called stub) is disposed in a branch of the circuit via which the capacitive element is discharged. The inverting unit is disposed between the wavelength-selective reflecting unit and the/each diode switch. The wavelength-selective reflecting unit is reflecting for a frequency range comprising the frequency of the carrier signal, and is conducting for all other frequencies. The inverting unit is formed e.g. as a lambda/4 line. The inverting unit and the wavelength-selective reflecting unit cooperate as a short-circuit for a certain RF frequency range. The frequency range comprises the frequency of the carrier signal and other frequencies in the region between 5xc2x710xe2x88x921 to 2 times the frequency of the carrier signal.
A further solution of the object of the present invention is based on a method for pulse modulation of a carrier signal of the above-mentioned type, wherein at least one capacitive element is charged and the discharge current of the at least one capacitive element is used for controlling the at least one diode switch.
Further features, possible applications and advantages of the invention can be extracted from the following description of embodiments of the invention which are shown in the drawing. All features described or shown define, individually or collectively in any arbitrary combination, the object of the invention independent of their concise articulation in the claims or their dependencies and independent of their formulation or representation in the description or in the drawing.